Formable aluminum alloy sheet

ABSTRACT

The present invention provides an aluminum alloy sheet for forming which is a high-Mg-content Al—Mg alloy sheet reduced in β-phase precipitation and improved in press formability. This aluminum alloy sheet for forming comprises an Al—Mg alloy containing 6.0-15.0 mass % Mg. In each of square regions, each side of which has the dimension of the whole sheet width (W), that are set in a surface of the alloy sheet, the concentration of Mg is measured at width-direction measurement points, Px, set at given intervals a and b respectively in the sheet-width direction and the sheet-length direction, and the average of the values of Mg concentration measured at the plurality of width-direction measurement points (Px) is taken as a width-direction average Mg concentration (Co). The concentration of Mg is measured at a plurality of thickness-direction measurement points (Py) set at a given interval in the sheet-thickness direction throughout the whole sheet thickness with respect to the plurality of width-direction measurement points (Px), and the average of the values of Mg concentration measured at the plurality of thickness-direction measurement points (Py) is taken as a thickness-direction average Mg concentration (Ci). The absolute value of the degree of regional Mg segregation (X) defined by the difference (Ci−Co) between the thickness-direction average Mg concentration (Ci) and the width-direction average Mg concentration (Co) is 0.5 mass % or less at most and is 0.1 mass % or less on average.

TECHNICAL FIELD

The present invention relates to a formable aluminum alloy sheet which is an Al—Mg alloy sheet containing Mg in a high content and has satisfactory formability.

BACKGROUND ART

As is well known, a variety of aluminum alloy sheets has been generally widely used in transportation machines such as automobiles, ships, aircraft, and vehicles; machines; electric products; construction materials; structures; optical appliances; and members or parts of wares, according to properties of respective alloy categories. Such aluminum alloy sheets often formed typically through stamping into the members and parts for use in these applications. Of aluminum alloys, Al—Mg alloys being in good balance between strength and ductility are advantageous for satisfactory formability. The Al—Mg alloys are represented by alloys prescribed in Japanese Industrial Standards (JIS) A 5052 and A 5182. These Al—Mg alloy sheets, however, have inferior ductility and thereby have inferior formability to customary cold-rolled steel sheets. As solutions to these disadvantages, chemical compositions of Al—Mg alloy sheets and optimization of manufacturing conditions of them have been studied.

Typically, an Al—Mg alloy, when having an increased Mg content of more than 6 percent by mass, preferably more than 8 percent by mass, may have better strength-ductility balance. However, a sheet of such a high-Mg-content Al—Mg alloy is hardly industrially manufactured by a common manufacturing method of casting a material into an ingot typically through direct chill (DC) casting, soaking the ingot, and hot-rolling the soaked ingot. This is because Mg is segregated in the ingot upon casting and causes the Al—Mg alloy to have significantly reduced ductility and to be liable to suffer from cracking upon common hot rolling process.

A possible solution to avoid the temperature range at which cracking occurs is hot rolling at a low temperature. However, it is also difficult to manufacture the high-Mg-content Al—Mg alloy sheet through such low-temperature hot rolling. This is because the high-Mg-content Al—Mg alloy material has remarkably high resistance to deformation upon the low-temperature rolling, and this extremely restricts sizes of products to be manufactured in view of performance of current rolling mills. Independently, a technique of adding a third element such as Fe or Si has been proposed so as to allow an Al—Mg alloy to contain Mg in a higher content. However, the material Al—Mg alloy, if containing the third element in a high content, tends to suffer from coarse intermetallic compounds and causes the aluminum alloy sheet to have low ductility. Increase in Mg content therefore has a ceiling according to this technique, and it is difficult to allow an Al—Mg alloy to contain Mg in a content of more than 8 percent by mass.

As possible solutions to these issues, various techniques have been proposed so as to manufacture high-Mg-content Al—Mg alloy sheets by a twin-roll continuous casting or another continuous casting process.

For example, Patent Literature (PTL) 1 describes an aluminum alloy sheet for automobiles, which is manufactured by twin-roll continuous casting and is a high-Mg-content Al—Mg alloy sheet having a Mg content of 6 to 10 percent by mass, in which Al—Mg intermetallic compounds have an average size of 10 μm or less.

PTL 2 describes an aluminum alloy sheet for automotive body sheets, which is manufactured by continuous casting and is an Al—Mg alloy sheet having a Mg content of 2.5 to 8 percent by mass. The aluminum alloy sheet contains Al—Mg intermetallic compounds having sizes of 10 μm or more in a number density of 300/mm² or less and has an average grain size of 10 to 70 μm.

PTL 3 describes an Al—Mg alloy sheet which is manufactured by twin-roll continuous casting and has a Mg content of 8 to 14 percent by mass. When Mg concentrations are measured throughout a thickness direction of the Al—Mg alloy sheet and averaged to give an average, absolute values of differences (deviation widths) between the average Mg concentration and the respective Mg concentrations have a maximum of 4 percent by mass or less and an average of 0.8 percent by mass or less. This may inhibit the precipitation of Al—Mg intermetallic compounds.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. H07-252571

PTL 2: JP-A No. H08-165538

PTL 3: JP-A No. 2007-77485

SUMMARY OF INVENTION Technical Problem

Al—Mg intermetallic compounds precipitated upon casting often cause fracture upon stamping, as described in PTL1 and PTL 2. Downsizing of Al—Mg intermetallic compounds (also referred to as “β phase(s)”) or reduction in amounts of coarse β phases is therefore effective for increasing stamping performance of a high-Mg-content Al—Mg alloy sheet. According to the techniques described in PTL 1 and PTL 2, Al—Mg intermetallic compounds, which will precipitate upon casting, are suppressed by performing cooling at a high rate (at a high casting rate) in the casting step. With an increasing Mg content in an Al—Mg alloy sheet, however, it becomes difficult to reduce the β phases to such an extent as not to adversely affect the stamping performance, when the control of cooling rate in the casting step is employed alone.

The technique disclosed in PTL 3 employs control of soaking and final annealing conditions to reduce the level of Mg segregation (Mg concentration distribution) throughout a thickness direction of the sheet, to suppress the precipitation of Al—Mg intermetallic compounds (β phases) caused by segregation of Mg (nonuniformity in Mg concentration). However, such a high-Mg-content Al—Mg alloy sheet manufactured by a customary twin-roll continuous casting process suffers from Mg segregation also in a width direction. For this reason, it is disadvantageously difficult to reduce β phases of the high-Mg-content Al—Mg alloy sheet to such an extent as not to adversely affect the stamping performance, when the reduction in level of Mg segregation throughout the thickness direction is employed alone.

Accordingly, demands have been made to provide a technique for suppressing β phases of a high-Mg-content Al—Mg alloy sheet to such an extent as not to adversely affect the stamping performance, instead of, or in addition to, the control of cooling rate in a casting step and/or the suppression of the level of Mg segregation in the thickness direction by the control of conditions for soaking and final annealing subsequent to the casting step.

The present invention has been made to solve such problems, and an object thereof is to provide a formable aluminum alloy sheet which is a high-Mg-content Al—Mg alloy sheet, less suffers from the precipitation of β phases inside thereof, and has better stamping performance (stamping formability).

Solution to Problem

To achieve the object, the present invention provides a formable aluminum alloy sheet which contains Mg in a content of 6.0 percent by mass or more and 15.0 percent by mass or less and further contains Al and impurities. The aluminum alloy sheet has a width-direction average Mg concentration of Co, where the width-direction average Mg concentration (Co) is an average of Mg concentrations measured at plural width-direction measurement points arranged at predetermined spacings in a width direction and in a length direction of the sheet, respectively, in a square region set on a surface of the formable aluminum alloy sheet, each side of the square region has the dimension of an entire sheet width. The aluminum alloy sheet has thickness-direction average Mg concentrations of Ci for the plural width-direction measurement points, respectively, where the thickness-direction average Mg concentrations (Ci) are each an average of Mg concentrations measured at plural thickness-direction measurement points arranged in a thickness direction of the sheet at a predetermined spacing throughout an entire sheet thickness. The aluminum alloy sheet has regional Mg segregation degrees of X each defined as a difference (Ci−Co) between each of the thickness-direction average Mg concentrations (Ci) and the width-direction average Mg concentration (Co), in which absolute values of the regional Mg segregation degrees (X) have a maximum of 0.5 percent by mass or less and an average of 0.1 percent by mass or less.

The formable aluminum alloy sheet, as having the above configuration, has regional Mg segregation degrees (X) having a maximum and an average controlled to predetermined levels or less, which regional Mg segregation degrees (X) are each defined as a difference between each of the thickness-direction average Mg concentrations (Ci) and the width-direction average Mg concentration (Co). The formable aluminum alloy sheet thereby less undergoes Mg segregation in the entire sheet, namely, both in the thickness direction and in the width direction; and less suffers from the precipitation of β phases inside thereof, and nonuniform deformation and resulting strain concentration upon forming.

In a preferred embodiment of the present invention, the aluminum alloy sheet has thickness-direction Mg concentrations of Ct, where the thickness-direction Mg concentrations are measured in a thickness direction at a predetermined spacing throughout the entire sheet thickness for at least one of the width-direction measurement points upon determination of the regional Mg segregation degrees (X); and the aluminum alloy sheet has, in addition to the regional Mg segregation degrees (X), thickness-direction Mg segregation degrees of Y each defined as a difference (Ct−Ci) between each of the thickness-direction Mg concentrations (Ct) and a thickness-direction average Mg concentration (Ci) corresponding to the at least one width-direction measurement point, in which absolute values of the thickness-direction Mg segregation degrees (Y) have a maximum of 4 percent by mass or less and an average of 0.8 percent by mass or less.

The formable aluminum alloy sheet, when having the above configuration, has thickness-direction Mg segregation degrees (Y) having a maximum and an average controlled to predetermined levels or less, in addition to the regional Mg segregation degrees (X). The thickness-direction Mg segregation degrees (Y) are each defined as a difference between each of the thickness-direction Mg concentrations (Ct) and the thickness-direction average Mg concentration (Ci). The formable aluminum alloy sheet may thereby further less undergo Mg segregation and further less suffer from the precipitation of β phases inside thereof, and nonuniform deformation and resulting strain concentration upon forming.

In another preferred embodiment of the present invention, the formable aluminum alloy sheet contains Mg in a content of more than 8 percent by mass and less than or equal to 14 percent by mass.

The formable aluminum alloy sheet, when having the above configuration, has a Mg content within the specific range, may thereby have higher strengths and better ductility and less suffer from the precipitation of β phases inside thereof.

In yet another preferred embodiment of the formable aluminum alloy sheet according to the present invention, the impurities include at least one element selected from the group consisting of Fe in a content of 1.0 percent by mass or less, Si in a content of 0.5 percent by mass or less, Ti in a content of 0.1 percent by mass or less, B in a content of 0.05 percent by mass or less, Mn in a content of 0.3 percent by mass or less, Cr in a content of 0.3 percent by mass or less, Zr in a content of 0.3 percent by mass or less, V in a content of 0.3 percent by mass or less, Cu in a content of 1.0 percent by mass or less, and Zn in a content of 1.0 percent by mass or less.

The formable aluminum alloy sheet, when having the above configuration, is controlled on the contents of Fe and Si acting as impurities, may thereby less suffer from the precipitation of intermetallic compounds inside thereof, and have better fracture toughness and better stamping performance. The intermetallic compounds herein are Al—Mg—(Fe, Si) and other Al—Mg intermetallic compounds; and other intermetallic compounds than Al—Mg intermetallic compounds, such as Al—Fe and Al—Si intermetallic compounds. This aluminum alloy sheet is also controlled on the contents of Ti, B, Mn, Cr, Zr, V, Cu, and Zn acting as impurities may be protected from adverse effects on stamping performance.

Advantageous Effects of Invention

The formable aluminum alloy sheet according to the present invention less undergoes Mg segregation, less suffers from the resulting formation of β phases, and can exhibit superior stamping performance. The formable aluminum alloy sheet can have further better stamping performance when having a Mg content controlled within a narrower range or by containing, in addition to Mg, at least one element selected from Fe, Si, Ti, B, Mn, Cr, Zr, V, Cu, and Zn in a controlled content.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 depicts plural measurement points for Mg concentrations to be employed upon determination of Mg segregation degrees of a formable aluminum alloy sheet according to the present invention, in which view (a) is a plan view, and view (b) is a cross-sectional view taken along the line A-A in view (a).

[FIG. 2] FIG. 2 is a cross-sectional view schematically illustrating a structure of continuous strip casting equipment for use in the manufacture of a formable aluminum alloy sheet according to the present invention.

[FIG. 3] FIG. 3 is a cross-sectional view schematically illustrating a continuous strip casting equipment for use in the manufacture of a formable aluminum alloy sheet according to the present invention.

[FIG. 4] FIG. 4 is a graph illustrating calculation results of regional Mg segregation degrees in a formable aluminum alloy sheet which satisfies conditions specified in the present invention.

[FIG. 5] FIG. 5 is a graph illustrating calculation results of regional Mg segregation degrees in a formable aluminum alloy sheet which does not satisfy the conditions specified in the present invention.

[FIG. 6] FIG. 6 is a graph illustrating calculation results of thickness-direction Mg segregation degrees in the formable aluminum alloy sheet which satisfies the conditions specified in the present invention.

[FIG. 7] FIG. 7 is a graph illustrating calculation results of thickness-direction Mg segregation degrees in the formable aluminum alloy sheet which does not satisfy the conditions specified in the present invention.

DESCRIPTION OF EMBODIMENTS

Formable aluminum alloy sheets according to embodiments of the present invention will be illustrated in detail below.

A formable aluminum alloy sheet according to an embodiment of the present invention (hereinafter also simply referred to as an “aluminum alloy sheet”) is a sheet of an aluminum alloy containing Mg in a high content and has regional Mg segregation degrees X controlled to predetermined values or less, which regional Mg segregation degrees X are each defined by a width-direction average Mg concentration Co and a thickness-direction average Mg concentration Ci.

Initially, a chemical composition of the aluminum alloy sheet according to the present embodiment will be illustrated on significance of respective alloy elements and on reasons why the contents are specified.

The aluminum alloy sheet according to the present embodiment includes an aluminum alloy containing Mg in a content of 6.0 percent by mass or more and 15.0 percent by mass or less and further containing Al and impurities, i.e., a high-Mg-content Al—Mg alloy. In a preferred embodiment, the aluminum alloy sheet includes a high-Mg-content Al—Mg alloy containing, as another element than Mg and as an impurity, at least one element selected from the group consisting of Fe in a content of 1.0 percent by mass or less, Si in a content of 0.5 percent by mass or less, Ti in a content of 0.1 percent by mass or less, B in a content of 0.05 percent by mass or less, Mn in a content of 0.3 percent by mass or less, Cr in a content of 0.3 percent by mass or less, Zr in a content of 0.3 percent by mass or less, V in a content of 0.3 percent by mass or less, Cu in a content of 1.0 percent by mass or less, and Zn in a content of 1.0 percent by mass or less.

Mg

Magnesium (Mg) element is an important alloy element to increase strengths and ductility of the aluminum alloy sheet. An aluminum alloy sheet containing Mg in a content of less than 6.0 percent by mass may have insufficient strengths and ductility and may fail to exhibit characteristics as a high-Mg-content Al—Mg alloy, resulting in insufficient stamping performance. In contrast, an aluminum alloy sheet containing Mg in a content of more than 15.0 percent by mass may be difficult to have Mg segregation, i.e., the regional Mg segregation degrees, controlled within the predetermined range even when its manufacturing method and conditions are controlled. An aluminum alloy sheet having regional Mg segregation degrees not controlled within the predetermined ranges may suffer from precipitation of larger amounts of β phases, thereby have remarkably inferior stamping performance, undergo greater work hardening, and thereby have insufficient cold rolling properties. To avoid these, the aluminum alloy sheet has a Mg content of 6.0 percent by mass or more and 15.0 percent by mass or less, and preferably more than 8 percent by mass and less than or equal to 14 percent by mass.

Fe and Si

Iron (Fe) and silicon (Si) elements should be minimized in amounts. Fe and Si precipitate as Al—Mg intermetallic compounds typically containing Al—Mg—(Fe, Si); and other intermetallic compounds than Al—Mg intermetallic compounds, such as Al—Fe and Al—Si intermetallic compounds. The aluminum alloy sheet, if having an Fe content of more than 1.0 percent by mass or a Si content of more than 0.5 percent by mass, may suffer from precipitation of such intermetallic compounds in excessively large amounts and thereby have significantly deteriorated fracture toughness and formability, resulting in significantly inferior stamping performance. To avoid these, the aluminum alloy sheet has an Fe content of 1.0 percent by mass or less and preferably 0.5 percent by mass or less, and a Si content of 0.5 percent by mass or less and preferably 0.3 percent by mass or less.

Ti, B, Mn, Cr, Zr, V, Cu, and Zn

Titanium (Ti) and boron (B) have the effect of allowing a cast strip (ingot) to have a finer structure. Manganese (Mn), chromium (Cr), zirconium (Zr), and vanadium (V) have the effect of allowing a rolled sheet to have a finer structure; and copper (Cu) and zinc (Zn) also have the effect of allowing a sheet to have higher strengths. To exhibit these effects, one or more of these elements may be contained within a range not adversely affecting the stamping performance which features the alloy sheet according to the present invention. Preferred contents of these elements to be acceptable are 0.1 percent by mass or less of Ti, 0.05 percent by mass or less of B, 0.3 percent by mass or less of Mn, 0.3 percent by mass or less of Cr, 0.3 percent by mass or less of Zr, 0.3 percent by mass or less of V, 1.0 percent by mass or less of Cu, and 1.0 percent by mass or less Zn.

Next, detailed description will be made with reference to FIG. 1 on a width-direction average Mg concentration Co, thickness-direction average Mg concentrations Ci, and regional Mg segregation degrees X of the aluminum alloy sheet, which regional Mg segregation degrees X are defined by the width-direction average Mg concentration Co and the thickness-direction average Mg concentrations Ci.

Width-Direction Average Mg Concentration Co

With reference to FIG. 1( a), the width-direction average Mg concentration Co may be determined in the following manner. Initially, a square region each side of which has the dimension of the entire sheet width W is set on a surface of an aluminum alloy sheet 60. Plural width-direction measurement points Px are set at a predetermined spacing “a” in a width direction and at a predetermined spacing “b” in a length direction in the square region, namely, a region surrounded by sides of the entire sheet width W and sides of the sheet length L having the same dimension with the entire sheet width W. Mg concentrations are measured at the plural width-direction measurement points Px in the surface of the aluminum alloy sheet 60. An average of the measured Mg concentrations is defined as a width-direction average Mg concentration Co which acts as an index for the level of Mg segregation on the surface in the width direction of the aluminum alloy sheet 60. The Mg concentrations may be measured by preparing an electron probe X-ray microanalyzer (EPMA) capable of performing linear analysis, and scanning the aluminum alloy sheet 60 in the width direction using the EPMA.

Reproducibility in measurement of level of Mg segregation in the width direction of the aluminum alloy sheet 60 may be obtained by setting a square region, each side of which has the dimension of the entire sheet width W, on a surface of the aluminum alloy sheet 60, and measuring Mg concentrations in the region on the surface of the aluminum alloy sheet 60. The width-direction measurement points Px are preferably set in the region in a number (number of points) of 5 or more in the width direction excluding sheet edges and 5 or more in the length direction, i.e., a total of 25 or more. The spacing “a” in the width direction and the spacing “b” in the length direction of the sheet may be set so as to set the width-direction measurement points Px in a number of 25 or more. In a more preferred embodiment, the spacing “b” in the length direction may be set so as to be 0.5 to 2 times the spacing “a” in the width direction.

Thickness-Direction Average Mg Concentration Ci

With reference to FIG. 1( b), the thickness-direction average Mg concentrations Ci are calculated in the following manner. Initially, plural thickness-direction measurement points Py are set at a predetermined spacing “c” in a thickness direction throughout the entire sheet thickness T. They are set for all the width-direction measurement points Px in the region. Mg concentrations (i.e., after-mentioned thickness-direction Mg concentrations Ct) at corresponding sheet depth positions are measured for each thickness-direction measurement point Py. An average of the measured Mg concentrations is defined as a thickness-direction average Mg concentration Ci which acts as an index for level of Mg segregation in the thickness direction (depth direction) of the aluminum alloy sheet 60. The Mg concentrations may be measured with an EPMA as above. Specifically, Mg concentrations at the respective thickness positions throughout the entire sheet thickness T in the region may be measured by scanning, in the thickness direction, cross sections of the aluminum alloy sheet cut in the width direction.

The spacing “c” in the thickness direction of the sheet is preferably set to 0.2 mm or less for ensuring reproducibility of the level of Mg segregation in the thickness direction of the aluminum alloy sheet 60. An initial measurement point of the thickness-direction measurement points Py is the corresponding width-direction measurement point Px at which a Mg concentration has already been measured.

Regional Mg Segregation Degrees X

The regional Mg segregation degrees X are each defined as a difference (Ci−Co) between each of the thickness-direction average Mg concentrations (Ci) and the width-direction average Mg concentration (Co), and act as an index for the level of Mg segregation in the entire aluminum alloy sheet 60, namely, both in the thickness direction and in the width direction. In the aluminum alloy sheet 60 according to the present invention, absolute values of the regional Mg segregation degrees X have a maximum of 0.5 percent by mass or less and an average of 0.1 percent by mass or less. The width-direction average Mg concentration Co, thickness-direction average Mg concentrations Ci, and regional Mg segregation degrees X may be controlled by the chemical composition of the aluminum alloy sheet 60 and after-mentioned manufacturing conditions thereof, typified by cooling conditions upon casting, thickness or facing amount (stock removal) of a cast strip, soaking conditions, and final annealing conditions.

If the regional Mg segregation degrees X have a large positive maximum and/or a large positive average, β phases are liable to precipitate due to Mg segregation. Such β phases will cause fracture and, when increased, adversely affect strengths and elongation of the aluminum alloy sheet, resulting in insufficient formability. If the regional Mg segregation degrees X have a large negative maximum and/or a large negative average, the aluminum alloy sheet may include a large number of local areas with significantly low Mg concentrations. The areas with significantly low Mg concentrations have low strengths. Therefore only the areas with low Mg concentrations preferentially deform upon tensile deformation in forming, resulting in nonuniform deformation. This may lead to local or partial concentration of strain upon forming and cause the aluminum alloy sheet to have insufficient formability particularly due to inferior elongation.

For these reasons, an aluminum alloy sheet 60 may have insufficient formability if having a maximum of more than 0.5 percent by mass and/or an average of more than 0.1 percent by mass in absolute values of regional Mg segregation degrees X, i.e., if not satisfying either one or both of the conditions specified in the present invention.

The aluminum alloy sheet 60 according to the present invention is preferably controlled on, in addition to the regional Mg segregation degrees X, thickness-direction Mg segregation degrees Y to predetermined levels or less. The thickness-direction Mg segregation degrees Y are each defined by a thickness-direction Mg concentration Ct and a thickness-direction average Mg concentration Ci.

Thickness-Direction Mg Concentrations Ct and Thickness-Direction Average Mg Concentration Ci

The thickness-direction Mg concentrations Ct are Mg concentrations measured at the plural thickness-direction measurement points Py illustrated in FIG. 1( b), as described above. The thickness-direction average Mg concentration Ci is an average of the measured thickness-direction Mg concentrations Ct.

The thickness-direction Mg concentrations Ct and the thickness-direction average Mg concentration Ci are Mg concentrations in the thickness direction of the sheet measured for at least one width-direction measurement point Px in the region, as measured upon determination of the regional Mg segregation degrees X. The thickness-direction Mg concentrations Ct are preferably determined by measuring Mg concentrations for one width-direction measurement point Px in the center part of sheet width; and are more preferably determined by measuring Mg concentrations for three width-direction measurement points Px including one in the center part and two in the vicinities of both ends of sheet width, and averaging the measured values.

Thickness-Direction Mg Segregation Degrees Y

The thickness-direction Mg segregation degrees Y are each defined as a difference (Ct−Ci) between a thickness-direction Mg concentration (Ct) and the thickness-direction average Mg concentration (Q), and act as an index for the level of Mg segregation in the thickness direction of the aluminum alloy sheet 60. The thickness-direction Mg segregation degrees Y, when used in combination with the regional Mg segregation degrees X, can satisfactorily reproduce the level of Mg segregation throughout the entire aluminum alloy sheet 60. In the aluminum alloy sheet 60 according to the present invention, absolute values of the thickness-direction Mg segregation degrees Y preferably have a maximum of 4 percent by mass or less and an average of 0.8 percent by mass or less. The thickness-direction Mg concentrations Ct, thickness-direction average Mg concentrations Ci, and thickness-direction Mg segregation degrees Y may be controlled by the chemical composition of the aluminum alloy sheet 60 and the manufacturing conditions thereof, typified by cooling conditions upon casting, thickness or facing amount (stock removal) of the cast strip, soaking conditions, and final annealing conditions.

If the thickness-direction Mg segregation degrees Y have a large positive maximum and/or a large positive average, β phases are liable to precipitate due to Mg segregation. Such β phases will cause fracture and, when increased, adversely affect strengths and elongation of the aluminum alloy sheet, resulting in insufficient formability. If the thickness-direction Mg segregation degrees Y have a large negative maximum and/or a large negative average, the aluminum alloy sheet may include a large number of local areas with significantly low Mg concentrations. The areas with significantly low Mg concentrations have low strengths. Therefore, only the areas with low Mg concentrations preferentially deform upon tensile deformation in forming, resulting in nonuniform deformation. This may lead to local or partial concentration of strain upon forming and cause the aluminum alloy sheet to have insufficient formability particularly due to inferior elongation.

For these reasons, an aluminum alloy sheet 60 may exhibit insufficient formability if absolute values of the thickness-direction Mg segregation degrees Y have a maximum of more than 4 percent by mass and/or an average of more than 0.8 percent by mass. Specifically, an aluminum alloy sheet 60 not satisfying either one or both of the specific conditions may exhibit insufficient formability.

Average Grain Size

The aluminum alloy sheet according to the present invention preferably has an average grain size of 100 μm or less in its surface.

The aluminum alloy sheet, when including fine grains with an average grain size of 100 μm or less in its surface, may have better stamping performance. The aluminum alloy sheet, if including coarse grains with an average grain size of more than 100 μm, may be liable to have insufficient stamping performance, resulting in defects such as cracking and orange peel surfaces upon forming. In contrast, the aluminum alloy sheet, if including excessively fine grains with an excessively small average grain size, may suffer from stretcher strain (SS) marks upon stamping, which marks are peculiar to 5xxx aluminum alloy sheets. To avoid this, the aluminum alloy sheet preferably has an average grain size of 20 μm or more.

As used herein the term “grain size” refers to a greatest dimension of a grain in the length direction. The grain size is measured in the L direction according to a line intercept method by mechanically polishing the aluminum alloy sheet to 0.05 to 0.1 mm, electrolytically etching the surface of the polished sheet, and observing the etched surface with an optical microscope at a 100-fold magnification. In this measurement, one measurement line is set to have a length of 0.95 mm. Three lines are set per one field of view, and a total of five fields of view are observed. Thus, the total measurement line length is 0.95 mm times 15 mm.

Next, a method for manufacturing the aluminum alloy sheet will be illustrated.

The aluminum alloy sheet according to the present invention may be manufactured through a melting-casting step, a soaking step, a cold rolling step, and a final annealing step. The respective steps will be illustrated below.

{Melting-Casting Step}

The melting-casting step is a step of melting a high-Mg-content Al—Mg alloy having the aforementioned chemical composition to give a molten metal, and preparing a cast strip from the molten metal by a continuous strip casting process. The continuous strip casting process is preferably a continuous casting process using a fixed graphite mold.

The continuous casting process using a fixed graphite mold is performed with continuous strip casting equipment 10 as illustrated in FIG. 2. Initially, a molten metal 2 stored in a holding furnace 1 is poured through an inlet la into a continuous mold 3 (fixed graphite mold 4). Next, the molten metal 2 is solidified in the fixed graphite mold 4 which is cooled with a water-cooling jacket 5. This gives a cast strip 6 having a small thickness. The prepared cast strip 6 is carried out by rolls 7 to a subsequent step. This casting process employs cooling at a higher rate than that in the DC casting process and thereby provides a finer cast structure and contributes to better stamping performance. In addition, this casting process can give a cast strip having a relatively small thickness of about 5 mm and eliminates the need of hot rough rolling, hot finish rolling, and other steps performed after casting on customary DC ingots having thicknesses of 200 to 600 mm.

Cooling Rate

In the continuous casting process using a fixed graphite mold, cooling may be performed at a rate of 15° C./s provided that the cast strip 6 has a thickness of from 5 to 20 mm. Cooling, if performed at an excessively low rate, may cause the aluminum alloy sheet to suffer from high level of Mg segregation, and this may impede the control of Mg segregation degrees within the ranges specified in the present invention to fail to suppress precipitation of β phases induced by Mg segregation. In this connection, the regional Mg segregation degrees X and thickness-direction Mg segregation degrees Y are hereinafter also synthetically referred to as “Mg segregation degrees.” In addition, β phases are generally liable to be coarse and precipitate in large amounts. This may highly possibly cause the aluminum alloy sheet to have significantly inferior stamping performance.

It is difficult to measure the cooling rate directly. The cooling rate may therefore be determined from a dendrite arm spacing (secondary dendrite arm spacing: DAS) of the cast strip 6 after casting by a known method, which is typified by a method described in The Japan Institute of Light Metals (ed.): “Measuring method of dendrite arm spacing and cooling rate of aluminum alloy,” issued on Aug. 20, 1988. Specifically, the cooling rate may be determined by measuring an average spacing “d” of adjacent secondary dendrite arms (secondary arms) in the cast structure of the cast strip 6 according to the line intercept method (in three or more fields of view at ten or more intercepting points). The cooling rate is then determined according to the following expression: d=62×C−0.337, wherein d represents the secondary dendrite arm spacing (mm); and C represents the cooling rate (° C./s). The cooling rate can therefore also be said as a solidification rate.

Pouring Temperature

The continuous casting process using a fixed graphite mold may employ pouring of the molten metal 2 into the fixed graphite mold 4 at a temperature higher than the liquidus temperature by 50° C. or more and 250° C. or less and preferably at a temperature higher than the liquidus temperature by 100° C. or more and 150° C. or less. Pouring, if performed at a temperature below the temperature higher than the liquidus temperature by 50° C. (liquidus temperature+50° C., may cause solidification of the molten metal within the mold to often cause rupture of the cast strip. Pouring, if performed at a temperature above the temperature of higher than the liquidus temperature by 250° C. (liquidus temperature+250° C.), may cause cooling upon casting to proceed slowly at a low rate and may thereby cause higher level of Mg segregation. This may impede the control of the Mg segregation degrees within the ranges specified in the present invention and may impede suppression of the precipitation of β phases and deterioration in formability due to large Mg segregation degrees.

Withdrawing Method

In the continuous casting process using a fixed graphite mold, the cast strip 6 may be backed by periodically rotating the rolls 7 in a direction opposite to the casting direction, for the stabilization of casting. The rolls 7 transport the cast strip 6 in the casting direction in a forward operation The backing may be performed at a stroke length of 0.5 mm or more and 5 mm or less, and preferably 1 mm or more and 3 mm or less. The castability is more stabilized when the cast strip is held for a holding time of 1 second or shorter before the backing.

If backing is performed at a stroke length of more than 5 mm, a segregated layer with a high Mg concentration generated in a surface of the cast strip 6 may penetrate into the strip to cause cracking of the cast strip at the site to thereby cause rupture. If backing is performed at a stroke length of less than 0.5 mm, a solid-liquid coexisting zone may not be compressed and may remain as susceptible to rupture, and this may cause the cast strip 6 to rupture in a region including the solid-liquid coexisting zone. To avoid these, the backing may be performed at a stroke length of from 0.5 mm or more and 5 mm or less.

Average Casting Rate

Casting of the molten metal 2 in the fixed graphite mold 4 in the continuous casting process using the fixed graphite mold may be performed at an average casting rate of 100 mm/min or more and 500 mm/min or less and preferably 250 mm/min or more and 350 mm/min or less. Casting, if performed at an average casting rate of less than 100 mm/min, may cause rapid solidification of the molten metal 2 in the vicinity of the inlet 1 a to increase the withdrawal resistance of the portion upon withdrawing by the rolls 7, and the resulting cast strip 6 may be susceptible to rupture. Casting, if performed at an average casting rate of more than 500 mm/min, may cause molten metal leakage in the vicinity of a cast strip outlet 4 a due to insufficient cooling.

Cast Strip Thickness

The cast strip 6 which has been continuously cast according to the continuous casting process using a fixed graphite mold may have a thickness in the range of 5 mm or more and 20 mm or less. If a cast strip 6 having a thickness of less than 5 mm is to be formed, the molten metal 2 may rapidly solidify in the vicinity of the inlet 1 a to increase the withdrawal resistance in the region upon withdrawing by the rolls 7, and the resulting cast strip 6 may be susceptible to rupture. If a cast strip 6 having a thickness of more than 20 mm is to be formed, cooling in the casting may be performed very slowly at an extremely low cooling rate to increase the level of Mg segregation. This may impede the control of Mg segregation degrees within the ranges specified in the present invention and impede suppression of precipitation of β phases induced by large Mg segregation degrees. In addition, β phases are generally liable to be coarse and precipitate in large amounts. This may highly possibly cause the aluminum alloy sheet to have significantly inferior stamping performance.

Facing

The continuous casting process using a fixed graphite mold preferably employs facing in which both sides of the prepared cast strip 6 are cut or shaved by a predetermined amount, because Mg segregation is liable to occur in the surfaces of the cast strip 6. The facing removes Mg-segregated regions on both sides of the strip and can thereby control the Mg segregation degrees within the ranges specified in the present invention. A depth of the Mg-segregated region corresponds to the back stroke length, and the facing may be performed in an amount corresponding to the back stroke length in the withdrawing.

The continuous strip casting process has been illustrated above by taking the continuous casting process using a fixed graphite mold as an example, but the process is not limited thereto. The continuous strip casting process may be any process, as long as capable of controlling the Mg segregation degrees of the aluminum alloy sheet within ranges specified in the present invention and may be, for example, a twin-roll continuous casting process.

The twin-roll continuous casting process may be performed with continuous strip casting equipment 100 as illustrated in FIG. 3. Initially a molten metal 300 is poured from a holding furnace 200 through a molten-metal-feeding nozzle 400 into a roll bite between a pair of rotating water-cooled copper molds (twin rolls 500) and solidified. The solidified metal immediately after solidification is rolled and quenched in the roll bite between the twin rolls 500 and thereby yields a cast strip 600 having a small thickness. The twin-roll continuous casting process is exemplified by a Hunter process and a continuous casting between cylinders (3C) process. The twin-roll continuous casting process can give a sheet (strip) having a relatively small thickness of 1 to 13 mm and eliminates the need for steps, such as hot rough rolling and hot finish rolling after casting, performed in manufacture of customary direct chill (DC) ingots (having thicknesses of 200 to 600 mm).

{Soaking Step}

The soaking step is a step of subjecting the cast strip 6 prepared in the preceding step to a predetermined soaking. The soaking is performed at a temperature of 400° C. or above and the liquidus temperature or below for a necessary duration. When the cast strip 6 prepared by the continuous strip casting process is soaked in a continuous heat-treating furnace, the heat treatment (soaking) is performed for a duration of roughly about one second (1 s) or shorter. The soaking reduces the level of Mg segregation and allows the aluminum alloy sheet to have Mg segregation degrees controlled within the ranges specified in the present invention.

There is a sufficient possibility of the generation of Al—Mg intermetallic compounds (β phases) both in temperature rise and cooling of the cast strip 6 in the soaking, if the rate of temperature rise and/or the cooling rate is excessively low. Such β phases are highly possibly generated at temperatures of the cast strip central part of from 200° C. to 400° C. during temperature rise; and at temperatures of from the soaking temperature down to 100° C. during cooling. To suppress the generation of β phases, heating up to the soaking temperature is preferably performed at an average rate of temperature rise of 5° C./s or more at temperatures of the cast strip central part of from 200° C. to 400° C.; and cooling down from the soaking temperature is preferably performed at an average cooling rate of 5° C./s or more at temperatures of the cast strip central part of from the soaking temperature down to 100° C.

{Cold Rolling Step}

The cold rolling step is a step of cold-rolling the soaked cast strip 6 to a thickness of a product sheet typically of 0.1 mm or more and 13 mm or less. The cold rolling converts the cast structure into a deformation structure. Accordingly, process annealing is preferably performed midway through the cold rolling so as to provide a cold rolling reduction in final cold rolling of 60% or less, when the cast strip 6 before cold rolling has a large thickness. The degree of conversion into a deformation structure as a result of cold rolling may vary also depending on the cold rolling reduction in cold rolling. For this reason, the cast structure may remain for the control of the metallic texture, but such residual cast structure is acceptable within a range not adversely affecting formability and mechanical properties.

{Final Annealing Step}

The final annealing step is a step of subjecting the cold-rolled sheet prepared in the preceding step to a predetermined final annealing. The final annealing step performs final annealing on the cold-rolled sheet at a temperature of 400° C. or above and below the liquidus temperature (° C.). The final annealing reduces the level of Mg segregation, allows the aluminum alloy sheet to have Mg segregation degrees within ranges specified in the present invention, and protects the aluminum alloy sheet from undergoing precipitation of β phases and from having insufficient stamping performance each due to Mg segregation.

A final annealing performed at a temperature of below 400° C. may highly possibly fail to provide solutionizing effects and may fail to reduce the level of Mg segregation effectively. To avoid these, the final annealing is preferably performed at a temperature of 450° C. or above. The sheet after the final annealing is preferably cooled at an average cooling rate as high as possible of 10° C./s or more at temperatures of from 500° C. down to 300° C. Cooling after the final annealing, if performed at a low average cooling rate of less than 10° C./s, may contrarily increase the level of Mg segregation during the cooling process. In this case, the resulting aluminum alloy sheet may fail to have Mg segregation degrees controlled within the ranges specified in the present invention and may possibly suffer from the precipitation of β phases and decrease in stamping performance due to large Mg segregation degrees. The average cooling rate is preferably 15° C./s or more.

EXAMPLES

Next, some working examples according to the present invention will be illustrated below.

Molten metals of Al—Mg alloys having chemical compositions given in Table 1 (Examples A, B, C, D, and E and Comparative Examples F and G) were cast under conditions given in Table 2 to give cast strips having thicknesses given in Table 2. The casting was performed by the continuous casting process using a fixed graphite mold or the twin-roll continuous casting process as mentioned above. The respective cast strips were selectively subjected to facing and soaking under conditions given in Table 2 and then cold-rolled to give cold-rolled sheets having a thickness of 1.0 mm or 11.0 mm without hot rolling. No process annealing was performed during the cold rolling. Next, the respective cold-rolled sheets were subjected to final annealing in a continuous annealing furnace at temperatures and cooling conditions given in Table 2 for a holding time at the annealing temperature of one second or shorter and thereby yielded aluminum alloy sheets for forming as Examples Nos. 1 to 5 and Comparative Examples Nos. 6 to 20. The aluminum alloy sheet for forming as Comparative Example No. 6 was prepared by a method through the twin-roll continuous casting process described in PTL 3.

The continuous casting process using a fixed graphite mold was performed at a back stroke length of 3 mm, an average casting rate of 300 mm/min, and a casting temperature (pouring temperature) of higher than the liquidus temperature by 140° C. The twin-roll continuous casting process was performed at a circumferential speed of the twin rolls of 70 m/min and at a pouring temperature for pouring the molten metal into a roll bite between the twin rolls of higher than the liquidus temperature by 20° C. This process was performed without lubrication of the surfaces of the twin rolls.

The liquidus temperatures of the respective alloys were calculated with a software for thermodynamic calculation Thermo-Calc Ver. R (Al-DATA Ver. 6).

The resulting formable aluminum alloy sheets (Examples Nos. 1 to 5 and Comparative Examples Nos. 6 to 22) were subjected to calculation and evaluation on regional Mg segregation degrees X and thickness-direction Mg segregation degrees Y according to the following procedures. The results are indicated in Table 2.

FIG. 4 depicts the calculated regional Mg segregation degrees X of Example No. 1; and FIG. 5 depicts the calculated regional Mg segregation degrees X of Comparative Example No. 6. FIG. 6 depicts the calculated thickness-direction Mg segregation degrees Y of Example 1; and FIG. 7 depicts the calculated thickness-direction Mg segregation degrees Y of Comparative Example No. 16.

Calculation and Evaluation of Regional Mg Segregation Degrees X

Initially, a square region with a dimension of one side of 100 mm was set on a surface of a sample formable aluminum alloy sheet. Next, five points excluding sheet edges were set in the width direction at a spacing of 16.6 mm (spacing “a”), and five points were set in the length direction at a spacing of 25 mm (spacing “b”), each in the region. Thus, a total of 25 width-direction measurement points Px (Nos. 1 to 25) was set (see FIG. 1( a)). Mg concentrations in the surface of the aluminum alloy sheet were measured at the respective measurement points and averaged to give a width-direction average Mg concentration Co.

Next, plural thickness-direction measurement points Py were set in the thickness direction at a spacing of 0.01 mm (spacing “c”) for each of the width-direction measurement points Px (Nos. 1 to 25) (see FIG. 1( b)). Mg concentrations of the aluminum alloy sheet at the respective measurement points (predetermined thickness positions (predetermined depth positions)) were measured and averaged to give a thickness-direction average Mg concentration Ci.

For each of the width-direction measurement points Px (Nos. 1 to 25), a regional Mg segregation degree X was calculated from the thickness-direction average Mg concentration Ci and the width-direction average Mg concentration Co, which regional Mg segregation degree X was defined as a difference between them (Ci−Co) (see FIGS. 4 and 5). The Mg concentrations were measured with an EPMA (JXA-8800RL, electron probe X-ray microanalyzer supplied by JEOL Ltd).

The regional Mg segregation degrees X were evaluated in the following manner. A sample having a maximum of absolute values of regional Mg segregation degrees X of 0.5 percent by mass or less was evaluated as satisfactory (◯); and a sample having the maximum of more than 0.5 percent by mass was evaluated as unsatisfactory (x). A sample having an average of absolute values of regional Mg segregation degrees X of 0.1 percent by mass or less was evaluated as satisfactory (◯); and a sample having the average of more than 0.1 percent by mass was evaluated as unsatisfactory (x).

Calculation and Evaluation of Thickness-Direction Mg Segregation Degrees Y

One (No. 13) of the width-direction measurement points (Nos. 1 to 25) was selected, and Mg concentrations measured at that point (No. 13) in the thickness direction (at the plural thickness-direction measurement points Py) were defined as thickness-direction Mg concentrations Ct. Thickness-direction Mg segregation degrees Y were calculated each as a difference (Ct−Ci) between each of the thickness-direction Mg concentrations Ct and the thickness-direction average Mg concentration Ci which had been calculated as an average of Mg concentrations measured at the respective (thickness-direction) measurement points. Thickness-direction measurement points Py, when positioned at a depth of 0.01 mm or 1.0 mm, are positioned in surfaces of the alloy sheet (see FIGS. 6 and 7).

The thickness-direction Mg segregation degrees Y were evaluated in the following manner. A sample having a maximum of absolute values of thickness-direction Mg segregation degrees Y of 4 percent by mass or less was evaluated as satisfactory (◯); and a sample having the maximum of more than 4 percent by mass was evaluated as unsatisfactory (x). A sample having an average of absolute values of thickness-direction Mg segregation degrees Y of 0.8 percent by mass or less was evaluated as satisfactory (◯); and a sample having the average of more than 0.8 percent by mass was evaluated as unsatisfactory (x).

Average Grain Size

Average grains sizes of the prepared formable aluminum alloy sheets (Examples Nos. 1 to 5 and Comparative Examples 6 to 22) were measured according to the aforementioned measuring method.

Examples Nos. 1 to 5 and Comparative Examples Nos. 6 to 10, 12 to 17, and 19 to 22 had average grain sizes in the range of 30 to 60 μm. Comparative Examples Nos. 11 and 18 had average grain sizes of more than 100 μm.

Evaluation of Stamping Performance

Stamping performances of the prepared formable aluminum alloy sheets (Examples Nos. 1 to 5 and Comparative Examples Nos. 6 to 22) were evaluated according to the following procedure. The results are indicated in Table 2.

Specimens were sampled from the alloy sheets and subjected to tensile tests to measure a tensile strength (TS (MPa)) and a total elongation (EL ( %)). The stamping performance was evaluated based on a strength-ductility balance defined as the product of TS and EL (TS×EL). A sample having a strength-ductility balance of 11000 or more was evaluated as accepted (◯); and a sample having a strength-ductility balance of less than 11000 was evaluated as rejected (x).

The specimens were sampled from each alloy sheet at arbitrary five points arranged across the longitudinal direction at a spacing of 100 mm or more. The measured tensile strengths and elongations of the five specimens per sample were respectively averaged to give a TS and an EL of the sample. The tensile tests were performed according to JIS Z 2201 using JIS No. 5 specimens. The specimens were prepared so that their longitudinal directions correspond to the rolling direction of the sample alloy sheet. The tensile tests were performed at a constant crosshead speed of 5 mm/min until the specimen was ruptured.

TABLE 1 Chemical composition (mass percent) Liquidus Category Code Mg Fe Si Ti B Mn Cr Zr V Cu Zn temperature (° C.) Examples A 10.0 0.25 0.21 0.01 0.002 — — — — — — 608 B 10.0 0.25 0.21 0.01 0.002 0.20 0.20 0.20 — 0.80 0.20 605 C 6.1 0.25 0.21 0.01 0.002 — — — — — — 628 D 8.1 0.25 0.21 0.01 0.002 — — — — — — 618 E 14.8 0.25 0.21 0.01 0.002 — — — — — — 582 Comparative F 5.8 0.25 0.21 0.01 0.002 — — — — — — 630 Examples G 15.3 0.25 0.21 0.01 0.002 — — — — — — 579

TABLE 2 Casting conditions Facing Cold rolling Cooling Cast strip Sheet Sheet Casting rate thickness thickness thickness Category Number Alloy process (° C./s) (mm) (mm) Soaking (mm) Examples 1 A graphite mold 400 10 4 450° C. for 1 s or shorter 1.0 2 B graphite mold 400 10 4 490° C. for 1 s or shorter 1.0 3 C graphite mold 400 10 4 400° C. for 1 s or shorter 1.0 4 D graphite mold 400 10 4 430° C. for 1 s or shorter 1.0 5 E graphite mold 400 10 4 460° C. for 1 s or shorter 1.0 Comparative 6 A twin-roll 400 5.5 — 450° C. for 1 s or shorter 1.0 Examples 7 F twin-roll 800 1.7 — 450° C. for 1 s or shorter 1.0 8 G twin-roll 800 1.7 — 450° C. for 1 s or shorter 1.0 9 A twin-roll 800 1.7 — — 1.0 10 B twin-roll 800 1.7 — — 1.0 11 A twin-roll 50 10 — 450° C. for 1 s or shorter 1.0 12 A twin-roll 800 1.7 — 450° C. for 1 s or shorter 1.0 13 A twin-roll 800 1.7 — 450° C. for 1 s or shorter 1.0 14 F graphite mold 400 10 4 450° C. for 1 s or shorter 1.0 15 G graphite mold 400 10 4 450° C. for 1 s or shorter 1.0 16 A graphite mold 400 10 4 — 1.0 17 B graphite mold 400 10 4 — 1.0 18 A graphite mold 5 25 19 450° C. for 1 s or shorter 11.0 19 A graphite mold 400 10 4 450° C. for 1 s or shorter 1.0 20 A graphite mold 400 10 4 450° C. for 1 s or shorter 1.0 21 A twin-roll 400 5.5 2 450° C. for 1 s or shorter 1.0 22 A graphite mold 400 10 — 450° C. for 1 s or shorter 1.0 Thickness-direction Mg Regional Mg Final annealing segregation degree segregation degree Stamping Cooling Maximum Average Maximum Average performance Temperature rate (mass (mass (mass (mass TSxEL Category Number (° C.) (° C./s) percent) percent) percent) percent) (MPa %) Examples 1 450 20.0 ∘ ∘ ∘ ∘ ∘ 2 450 20.0 ∘ ∘ ∘ ∘ ∘ 3 450 20.0 ∘ ∘ ∘ ∘ ∘ 4 450 20.0 ∘ ∘ ∘ ∘ ∘ 5 450 20.0 ∘ ∘ ∘ ∘ ∘ Comparative 6 450 20.0 ∘ ∘ x x x Examples 7 450 20.0 ∘ ∘ ∘ ∘ x 8 450 20.0 x x x x x 9 450 20.0 x x x x x 10 450 20.0 x x x x x 11 450 20.0 x x x x x 12 350 20.0 x x x x x 13 450 5.0 x x x x x 14 450 20.0 ∘ ∘ ∘ ∘ x 15 450 20.0 x x x x x 16 450 20.0 x x x x x 17 450 20.0 x x x x x 18 450 20.0 x x x x x 19 350 20.0 x x x x x 20 450 5.0 x x x x x 21 450 20.0 ∘ ∘ x x x 22 450 20.0 x x x x x

The results in Tables 1 and 2 demonstrate that Examples Nos. 1 to 5 satisfying the conditions specified in the present invention were superior in stamping performance to Comparative Examples Nos. 6 to 22 not satisfying the conditions in the present invention.

Specifically, Comparative Example No. 6, the alloy sheet described in PTL 3, failed to control Mg segregation degrees (regional Mg segregation degrees) within the ranges specified in the present invention and had poor stamping performance. Comparative Examples Nos. 7 and 14 had Mg segregation degrees controlled within the ranges specified in the present invention, but had Mg contents less than the lower limit, thereby had a poor strength-ductility balance and exhibited poor stamping performance. Comparative Examples Nos. 8 and 15 had Mg contents more than the upper limit, thereby had large Mg segregation degrees, and exhibited poor stamping performance. Comparative Examples Nos. 9, 10, 16, and 17 did not undergo soaking, thereby had large Mg segregation degrees, and exhibited poor stamping performance. Comparative Examples Nos. 11 and 18 underwent cooling in the casting performed at a low cooling rate, thereby had large Mg segregation degrees, and exhibited poor stamping performance. Comparative Examples Nos. 12 and 19 underwent final annealing performed at a low temperature, thereby had large Mg segregation degrees, and exhibited poor stamping performance. Comparative Examples 13 and 20 underwent final annealing performed at a low cooling rate, thereby had large Mg segregation degrees, and exhibited poor stamping performance. Comparative Example 21 underwent facing of both sides of a cast strip by 1.75 mm, which cast strip had been prepared by the twin-roll continuous casting process. This comparative example failed to have Mg segregation degrees (regional Mg segregation degrees) controlled within the ranges specified in the present invention and exhibited poor stamping performance. Comparative Example 22 did not undergo facing of both sides of a cast strip prepared by the continuous casting process using a fixed graphite mold, thereby had large Mg segregation degrees, and exhibited poor stamping performance.

While the present invention has been described with reference to embodiments and examples thereof, it will be understood that the invention is not limited to such specific embodiments thereof; and various modifications and changes may be made therein without departing from the spirit and scope of the invention as hereinafter claimed. The present application is based on Japanese Patent Application No. 2010-187756 filed on Aug. 25, 2010, the entire contents of which are incorporated herein by reference.

REFERENCE SIGNS LIST

a, b, c spacing

L sheet length

W sheet width

T sheet thickness

Px width-direction measurement point

Py thickness-direction measurement point

1 holding furnace

1 a inlet

2 molten metal

3 continuous casting mold

4 fixed graphite mold

4 a cast strip outlet

5 water-cooling jacket

6 cast strip

7 roll

10 continuous strip casting equipment

100 continuous strip casting equipment

200 holding furnace

300 molten metal

400 molten-metal-feeding nozzle

500 twin roll

600 cast strip 

1. An aluminum alloy sheet, comprising Mg in a content of 6.0 percent by mass or more and 15.0 percent by mass or less and further comprising Al and an impurity, the aluminum alloy sheet having: a width-direction average Mg concentration of Co, wherein the width-direction average Mg concentration, Co, is an average of Mg concentrations measured at a plurality of width-direction measurement points, wherein the plurality of width-direction measurement points is arranged at predetermined spacings in a width direction and in a length direction of the sheet, respectively, in a square region set on a surface of the aluminum alloy sheet, wherein each side of the square region has a dimension of an entire sheet width; a thickness-direction average Mg concentration, Ci, for each of the plurality of width-direction measurement points, wherein each Ci is an average of Mg concentrations measured at a plurality of thickness-direction measurement points arranged in a thickness direction of the sheet at a predetermined spacing throughout an entire sheet thickness; and regional Mg segregation degrees of X each defined as a difference, Ci−Co, between each thickness-direction average Mg concentration, Ci, and the width-direction average Mg concentration, wherein absolute values of the regional Mg segregation degrees, X, have a maximum of 0.5 percent by mass or less and an average of 0.1 percent by mass or less.
 2. The aluminum alloy sheet of claim 1, wherein the aluminum alloy sheet has thickness-direction Mg concentrations, Ct, wherein the thickness-direction Mg concentrations are measured in a thickness direction of the sheet at a predetermined spacing throughout the entire sheet thickness for the width-direction measurement points upon determination of the regional Mg segregation degrees, X; and thickness-direction Mg segregation degrees, Y, each defined as a difference, Ct−Ci, between each thickness-direction Mg concentration, Ct, and the thickness-direction average Mg concentration, Ci, corresponding to the respective width-direction measurement point, in which absolute values of the thickness-direction Mg segregation degrees, Y, have a maximum of 4 percent by mass or less and an average of 0.8 percent by mass or less.
 3. The aluminum alloy sheet of claim 1, wherein Mg is in a content of more than 8 percent by mass and less than or equal to 14 percent by mass.
 4. The aluminum alloy sheet of claim 1, wherein the impurity comprises at least one element selected from the group consisting of Fe in a content of 1.0 percent by mass or less, Si in a content of 0.5 percent by mass or less, Ti in a content of 0.1 percent by mass or less, B in a content of 0.05 percent by mass or less, Mn in a content of 0.3 percent by mass or less, Cr in a content of 0.3 percent by mass or less, Zr in a content of 0.3 percent by mass or less, V in a content of 0.3 percent by mass or less, Cu in a content of 1.0 percent by mass or less, and Zn in a content of 1.0 percent by mass or less.
 5. The aluminum alloy sheet of claim 2, wherein Mg is in a content of more than 8 percent by mass and less than or equal to 14 percent by mass.
 6. The aluminum alloy sheet of claim 2, wherein the impurity comprises at least one element selected from the group consisting of Fe in a content of 1.0 percent by mass or less, Si in a content of 0.5 percent by mass or less, Ti in a content of 0.1 percent by mass or less, B in a content of 0.05 percent by mass or less, Mn in a content of 0.3 percent by mass or less, Cr in a content of 0.3 percent by mass or less, Zr in a content of 0.3 percent by mass or less, V in a content of 0.3 percent by mass or less, Cu in a content of 1.0 percent by mass or less, and Zn in a content of 1.0 percent by mass or less.
 7. The aluminum alloy sheet of claim 1, comprising a fine grain.
 8. The aluminum alloy sheet of claim 7, having an average grain size of 20 μm or more and 100 μm or less on the surface.
 9. A method of producing the aluminum alloy sheet of claim 1, the method comprising: melt-casting by melting a high-Mg-content Al—Mg alloy to obtain a molten metal, and preparing a cast strip from the molten metal by continuous strip casting; soaking the cast strip in a continuous heat-treating furnace at a temperature of 400° C. or above and a liquidus temperature or below; cold rolling the cast strip to convert the cast strip into a deformation structure to obtain a cold-rolled sheet; and final annealing the cold-rolled sheet at a temperature of 400° C. or above and below the liquidus temperature.
 10. The method of claim 9, wherein the continuous strip casting comprises a fixed graphite mold.
 11. The method of claim 10, wherein the continuous strip casting comprises: pouring a molten metal stored in a holding furnace through an inlet into the fixed graphite mold; and solidifying the molten metal in the fixed graphite mold, by cooling with a water-cooling jacket.
 12. The method of claim 11, wherein the cooling is performed at a rate of 15° C./s wherein the cast strip has a thickness of from 5 to 20 mm.
 13. The method of claim 12, wherein the pouring is performed at a temperature higher than the liquidus temperature by 50° C. or more and 250° C. or less.
 14. The method of claim 10, wherein the continuous casting process is performed at an average casting rate of 100 mm/min or more and 500 mm/min or less.
 15. The method of claim 10, wherein the continuous casting comprises facing in wherein the cast strip is cut or shaved. 16 The method of claim 9, wherein the soaking is performed for a duration of one second or shorter.
 17. The method of claim 9, wherein the cold-rolling comprises cold-rolling the cast strip to a thickness of a product sheet of 0.1 mm or more and 13 mm or less.
 18. The method of claim 17, further comprising a process annealing performed midway through the cold rolling to obtain a cold rolling reduction in a final cold rolling of 60% or less.
 19. The method of claim 9, wherein the final annealing is performed at a temperature of 450° C. or above and below the liquidus temperature.
 20. The method of claim 9, further comprising cooling after final annealing at a cooling rate of 10° C./s or more at a temperature of 500° C. down to 300° C. 